1. Field of the Invention
The present invention relates to an image forming apparatus, method of controlling an image forming apparatus, and a storage medium.
2. Description of the Related Art
In recent years, an advance has been achieved in image forming technology used in an image formation apparatus configured to form an image on a sheet. As a result, an increase in image formation speed has been achieved. In such a high-speed image forming apparatus, a large number of sheets are discharged at a high speed from a main body of the image forming apparatus. Thus, in the technology of a sheet stacking apparatus that is connected to a main body of an image forming apparatus and that is configured to stack sheets discharged from the main body of the image forming apparatus, there is a need for a large capacity of stacking a large number of sheets.
Some recent sheet stacking apparatuses have a plurality of sheet stacking units disposed in one sheet stacking apparatus. Some such sheet processing apparatuses are configured such that when small-size sheets such as A4-size sheets are handled, it is allowed to stack sheets on each sheet stacking unit thereby achieving a high capacity without increasing the apparatus size (for example, see Japanese Patent Laid-Open No. 2008-87965).
In such a sheet stacking apparatus, when sheets of a large size such as A3 size are stacked, sheets are stacked on a plurality of sheet stacking units in such a manner that sheets are placed over the plurality of sheet stacking units thereby making it possible to stack large-size sheets.
In a case where a plurality of sheet stacking units are disposed in a single sheet stacking apparatus, the sheet stacking units are configured to be capable of moving up and down independently. To take out sheets stacked on one sheet stacking unit, the sheet stacking unit is lowered down to a taking-out position, while another sheet stacking unit is raised. This allows it to take out the first sheet stacking unit from the sheet stacking apparatus while sheets are stacked on the second sheet stacking unit. For example, as shown in FIG. 16, a user removes the stack of sheets together with the stacker tray 112a and the dolly 120 from the sheet stacking apparatus. Note that, in this state, the other stacker tray, i.e., the stacker tray 112b remains in the sheet stacking apparatus. Therefore, it is possible to continue stacking a large number of sheets on the sheet stacking apparatus without stopping the operation of discharging sheets to the sheet stacking apparatus. This allows an increase in a stacking efficiency.
However, when the sheet stacking unit taken out from the sheet stacking apparatus is returned into the sheet stacking apparatus, there is a possibility that the sheet stacking unit is put by mistake at a wrong location different from a correct location to which the sheet stacking unit should be returned. The result of this is that there is no sheet stacking apparatus at one location but a plurality of sheet stacking apparatuses are set in a vertically overlapping manner at the other location. In this case, only one sheet stacking unit is usable, and the capacity of the sheet stacking apparatus is not fully used. This results in a reduction in the stacking efficiency.